The present invention relates to a dry-sealing screw pump with two positive displacement spindle rotors that have external teeth and rotate in opposite directions, and which are used to deliver and compress gases, a gear wheel that is used to drive and synchronize the rotors being arranged on each of said rotors.
Dry-sealing pumps are becoming increasingly important, in particular in the domain of vacuum technology, for known wet-running vacuum systems such as the liquid rotary machines and rotary disc pumps, are being replaced ever more frequently by dry sealing pumps because of more stringent demands imposed by environmental-protection regulations and ever increasing operating and disposal costs, and because of increased demands for the purity of the delivery medium. These dry-sealing machines include screw pumps, claw pumps, diaphragm pumps, piston pumps, and scrolling machines, as well as Roots vacuum boosters. Common to all of these machines is the fact that they cannot satisfy today""s requirements with respect to reliability and robust construction, or with respect to installed size and weight combined with a low price.
Dry-sealing screw pumps are being used to an ever increasing extent in vacuum technology because, as typical twin-shaft displacement machines, they can realize the high compression capability that is specific to vacuum technology and do this very simply in that they achieve the required multi-stage configuration as a series of closed working chambers by way of the number of loops per spindle rotor. In addition, the non-contact (rotation) of the spindle rotors permits an increased speed of rotation for the rotors so that there is a simultaneous increase in both nominal throughput and charging efficiency relative to the installed size.
In the case of modern spindle vacuum pumps, which is to say in the case of screw pumps, the desired speeds of rotation for the spindle is in most instances clearly above the nominal speed of rotation of the asynchronous motors that are usually used for the drive systems because of their robust construction, so that a frequency converter or an up-stream gear drive has to be used in order to increase the speed of rotation. At these increased rotor speeds which, in most instances are clearly above 3000 r.p.m. (order of magnitude approximately 10,000 r.p.m.) the non-contact rotation of the two displacement spindles within the working chamber of the pump is absolutely essential.
Today, in the majority of cases, this is done with simple gear wheels. When this is done, because of the high speeds of rotation that are desired, this results in very high peripherals speeds for the teeth, with a simultaneously smaller specific flank loading, so that this type of machine is inclined to produce a so-called chatter because of the high dynamic factor.
A dry-sealing screw pump of this kind, with two gear wheels that mesh with each other for mechanical synchronization is described in DE 195 22 551 C2.
In addition to this synchronization tooth construction of the two spindle rotors, these must as a rule be preceded by a spur-gearing stage that is used to increase the rotational speed, so that in such a case a total of four front-toothed gears are needed. Up to now, it has not been possible to combine the two parallel gearing stages in a favorable manner, for direct engagement of a driving pinion in the synchronization toothing of the two spindle rotors that rotate in opposite directions would result in a driving gear wheel that is clearly too large by the factor of the desired increase of the rotational speed, because the working circle of the two synchronization gear wheels which are of equal size must of necessity correspond to the distance between the axes of the rotors used in the screw pump.
For this reason, it is the objective of the present invention to create a screw pump of the type described in the introduction hereto, in which the drive and the synchronization of the two spindle rotors for a fast running screw pump are as simple and as quiet as possible.
In order to achieve these objectives, which are apparently contradictory, the screw pump defined in the introduction hereto is characterised in that the toothing diameter of the two gear wheels for the two displacement spindle rotors hereinafter referred to as the rotors is smaller than the distance between the axes; in that a driving gear wheel engages in the two gear wheels of the rotors; and in that the meshing of the driving gear wheel with the driven gear wheels is realized in the form of a contrate like gear wheel. For the purposes of the specification, the term contrate like gear is used, and it should be understood that the this term includes the following types of gears: a spur gear with teeth parallel to the gears axis of rotation, a crown gear with teeth perpendicular to its axis of rotation, a spur/crown sear having both parallel and perpendicular teeth, and finally, a bevel gear having teeth at an angle to the gear""s axis of rotation as illustrated. When the generic term contrate like gear is not used, the appropriate more specific term, spur, crown, spur/crown, and bevel is used.
By employing this solution according to the present invention, the peripheral speed of the gear wheels for the rotors can be appreciably reduced and the specific flank loading on the teeth can be increased, so that the level of noise and the dynamic factor are both reduced. In addition, the desired increase in the rotational speed from the driving gear wheel to the spindle rotors can be achieved very simply by way of the diameter ratios and gear wheel tooth ratios of this driving gear wheel to the gear wheels of the rotors.
In addition, for all practical purposes, only three gear wheels are required and these are simple to install, which improves the cost situation. Furthermore, the xe2x80x9cconcept of the complete spindle unitxe2x80x9d can be implemented very simply:
Because the cost of the high rotor speeds, good balancing of the overall rotating rotor unit is expedient, which is to say that it is not sufficient to balance only the displacement rotor, for in the end, because of the subsequent addition of the extra elements such as rotor bearings, shaft seals, and gear wheels, even though each individual part is in itself well balanced the overall balance of these rotating units will be changed in such a way that the desired balanced mass of the total rotating unit can no longer be guaranteed. However, retrobalancing of a screw pump is costly. In the case of conventional synchronization teeth that engage in each other, the concept of the complete spindle unit can only be replaced by the double engagement of spindle delivery threads and synchronization gear wheels at great cost, because the intermediate bearing and expansion chamber shaft seals have to be set up and installed so as to be free of leaks.
Because of the solution according to the present invention, whereby the direct engagement of the gear wheels that are attached to the rotor with each other is avoided, assembly of the previously balanced unit is now made simpler so that the result achieved by the previous balancing is retained after installation.
Additionally, the solution according to the present invention makes it possible that the motor axis can be arranged in the same direction as the two spindle. rotor axes or at right angles thereto. This also reduces the amount of space required for the overall screw pump together with its motor, and facilitates the way in which the motor is cooled by the air flow, and can be adapted to any particular design features.
It is particularly useful if the driving gear wheel is greater than the two gear wheels that are fixed to the two spindle rotors. This is made possible mainly by the measure according to the present invention whereby the diameter of the toothing of the two gear wheels for the rotors is smaller than the distance between the axes of the two rotors, so that the driving gear wheel engages in both these gear wheels and can be of a corresponding size. This also makes it possible to realize the drive and synchronization for the two spindle rotors for a fast running screw pump in the simplest possible way and, at the same time, raise the speed of the two rotors by the desired factor, which can, for example, be between 1.5 and 4. Because they are of the diameter that is smaller than the distance between the axes, the two displacement spindle pump rotors can be acted upon together by a driving gear wheel with a tooth count that is greater by the factor of the desired rotational speed increase, so that the two spindle rotors can be driven in opposite directions at increased speed and be synchronized with each other at the same time. Because of this, the screw pump can be driven at a higher speed, whereby the compression power, charging efficiency, and thus the volumetric efficiency all are increased. At the same time, a more than proportionally higher pumping capacity is achieved from the identical machine size, so that specific costs-relative to volumetric flow-are reduced accordingly. The factor for increasing the speed can be the value of 1.5 to 4, as discussed heretofore, or if necessary be outside these limits relative to the standard rotational speed in the case of a direct drive. In an advantageous manner, it is possible to avoid the use of frequency converters-which are, generally speaking, costly-in order achieve the speed increase that is known per se.
The driving gear wheel can be mounted directly on the shaft of a drive motor. This also helps to make the overall drive system simpler.
In order to lubricate the point of engagement between the gear wheels, a lubricant can be fed internally onto the driving gear wheel. Because of the present invention""s arrangement and association of the gear wheels, lubrication can be greatly improved if lubricant is applied to the inside diameter of the toothed side of the driving gear wheel, in particular a contrate like gear wheel, when centrifugal force will distribute the lubricant to the point where the tooth flanks engage with each other; this will also help to reduce the level of noise that is generated.
The driving gear wheel and/or the gear wheel that is attached rigidly to the spindle rotor can be in the form of a contrate like gear wheel. According to the solution according to the present invention, the contrate like gear toothing or the configuration of the gear wheel(s) has already been designated. This can thus refer either to the driving gear wheel or to the gear wheels that are attached rigidly to the spindle rotor, or to all of the gear wheels. This means that the common engagement of the driving gear wheel with the two gear wheels on the rotors can be configured so as to save as much space as possible.
One useful configuration can be such that the driving gear wheel has one or two rings of teeth that are arranged like crown or crown/spur gears, and that these engage with the gear wheels that are attached rigidly to the rotor and configured as spur wheels. This means that there are relatively simple gear wheels on the rotors and that the crown or crown/spur gear like wheels of the driving gear wheel can engage in these, in that the toothed rims of the driving gear wheel are introduced into the space between the two driven gear wheels.
Another configuration and embodiment of the present invention can be such that as an internally and externally toothed spur gear, the crown/spur like gear wheel drives by way of its interior toothing a gear wheel on one rotor shaft, which is fixed to the rotor and configured as a spur gear, and by way of its external toothing drives the other gear wheel on the second rotor shaft, which is mounted rigidly on the rotor and configured as a spur gear wheel; it does this such that they rotate synchonously, in opposite directions, and at increased speeds of rotation. The crown/spur gear like driving gear wheel can thus be represented as or understood to be an internally and externally crown gear ring that with its two sets of toothing drives in each instance a classic spur gear in the desired manner and a crown gear in the desired manner as well, and so that they are synchronized in opposite directions and so that they are driven at an increased speed.
In order to save costs and reduce the amount of noise that it generates, a gear ring of this kind can be produced as a bundle of laminations.
Another embodiment of the present invention makes provision such that the teeth of the contrate like gear wheels are in each instance arranged on a cone, and the angle of the cone of the two sets of teeth that are provided on the driving gear wheel is relatively flat or pointed; this is sometimes called a beveled gear. This means that the gear wheels that are attached rigidly to the rotors are not spur gear wheels but rather also bevel gears with a relatively pointed angle of taper; under certain circumstances, this improves the manner in which the sets of teeth engage with each other.
Another possibility is such that the gear wheels that are connected to the rotors of the configured as contrate gear wheels, and the driving gear wheel is configured as a spur gear. This spur gear can then engage simultaneously on the areas of the contrate gear like gear rims that face each other, for example, when the toothing on the driving gear wheel can, however, be interrupted by an annular groove and confined to the area in which the teeth mesh with each other.
Mainly in the embodiments in which the driving gear wheel has two contrate gear like sets of teeth-in a more or less conical arrangement-it is in a more useful manner possible that the axis of rotation of the driving gear wheel be arranged at a right angle to the axis of rotation of the driven gear wheels and rotors; and in that the driving gear wheel that has a gear ring on each of the two sides that face away from each other meshes in the space between the two with driven gear wheels that are arranged in a common plane, and meshes with these gear wheels. This results in the above described space-saving arrangement and, at the same time, in simple production and assembly.
Mainly in a combination of individual or more of the features and measures described above, the result will be a drying-sealing screw pump, the drive for which is restricted to a few gear wheels, which permits mechanical synchronization and at the same time an increase in the rotational speed of the rotors, and does so in a simple manner. In addition, because of the simplification of the overall arrangement, it is possible to achieve good balancing of a rotating parts prior to assembly, so that the desired high rotor speeds can be implemented in a more favorable manner.